1. Field of the Invention
This invention relates generally to glass compositions having improved melting and refining characteristics and, more particularly, to methods of adjusting a glass composition to lower the temperature (s) of the melting and/or of forming viscosities and/or of devitrification preferably without substantially changing the temperature (s) of the bending and/or annealing viscosities of the glass, if so desired. The invention also relates to glass articles made from the glass compositions.
2. Technical Considerations
Glass manufacturers melt glass batch materials and refine the molten glass to form glass articles. For example, in a conventional float glass process, glass batch materials are heated in a furnace or melter to form a glass melt. The glass melt is poured onto a bath of molten tin, where the glass melt is formed and continuously cooled to form a float glass ribbon. The float glass ribbon is cooled and cut to form solid glass articles, such as flat glass sheets. The particular batch materials used and their relative amounts are selected based on the desired properties of the glass articles. Exemplary glass batch compositions are disclosed in U.S. Pat. Nos. 5,071,796; 5,837,629; 5,688,727; 5,545,596; 5,780,372; 5,352,640; and 5,807,417, just to name a few.
As will be appreciated by one of ordinary skill in the glass manufacturing art, glass composition properties can be defined based on their temperature and viscosity characteristics. For example, the xe2x80x9cmelting temperaturexe2x80x9d of a glass is conventionally defined as the temperature at which the glass has a viscosity of 100 poises, which is conventionally referred to as the temperature of the xe2x80x9clog 2xe2x80x9d viscosity (i.e., the logarithm of the viscosity of the glass in poises is 2). Similarly, the xe2x80x9cforming temperaturexe2x80x9d (log 4 viscosity), xe2x80x9cbending temperaturexe2x80x9d (log 7.6 viscosity), xe2x80x9cannealing temperature (log 13 viscosity), and xe2x80x9cstrain pointxe2x80x9d (log 14.5 viscosity), are conventionally defined as the temperatures at which the logarithms of the glass viscosity in poises are 4, 7.6, 13, and 14.5, respectively. The xe2x80x9cliquidus temperaturexe2x80x9d is that temperature at which the glass begins to devitrify, which can cause undesirable haziness in the glass product. The difference between the forming temperature and the liquidus temperature is known as the xe2x80x9cworking rangexe2x80x9d. It is generally desirable to have a working range spanning more if than 40xc2x0 F. (22xc2x0 C.).
Glass fabricators purchase flat glass sheets from glass manufacturers and process these glass sheets into various commercial products, such as architectural windows, mirrors, shower doors, automotive windows, insulating glass units, etc. Typically, this processing includes heating the flat glass sheets to bend the sheets and then controllably cool the sheets to anneal, temper, or heat strengthen the sheets. The bending, tempering and/or annealing temperatures for a particular type of glass are important economic factors in the fabrication process and cannot be easily changed without substantially altering the existing fabrication process, which would be expensive and time consuming.
Due to increased tonnage and quality demand for flat glass products, flat glass manufacturers are under pressure to increase their glass production while reducing the cost of manufacturing the glass. Many glass manufacturers are operating their glass furnaces at higher and higher throughput and temperatures to meet the increased demand for glass. However, this need to increase glass production has resulted in several problem areas. For example, the operating temperature of a conventional flat glass furnace is typically on the order of 2850xc2x0 F. (1564xc2x0 C.). As more glass batch material is processed through the furnace, more fuel is required to melt the increased amounts of glass batch materials in a shorter time period. This increased fuel usage adds significantly to the production cost of the glass sheets or articles and results in a decreased thermal efficiency for the melting operation. Further, running the melter at increased throughput and at elevated temperatures can also damage the melter refractories, such as by causing thermal and/or chemical damage to the silica crowns and breast walls, which can lead to premature failure or collapse of the melter superstructure and solid defects in the glass.
The forming temperature of glass made by the float glass process is maintained sufficiently high to avoid devitrification of the glass, thereby resulting in crystalline defects in the float glass product. With some glass compositions such higher forming temperatures can be problematic with an increase in the dissolution rate of sections of the float glass melter including sections which deliver the molten glass to the molten tin bath. For instance the service life of forming refractories of the melter could be decreased.
Therefore, it would be advantageous to provide glass manufacturers with a method of adjusting a glass composition (and thus the batch materials from which it is made) to provide a lower melting point and/or lower temperatures of forming and/or lower liquidus temperature. The former assists in decreasing fuel usage and potential damage to the melter while maintaining substantially the same bending and annealing temperatures as the starting glass composition. The latter can extend the service life of sections of the melter including the forming refractories.
The present invention provides a method of adjusting, e.g., lowering, the melting and/or forming temperatures and/or liquidus temperature of a glass composition. Such adjustments can avoid substantial changes to the bending and/or annealing temperatures of the glass. In one aspect of the invention directed to glass compositions containing calcium oxide (CaO) and magnesium oxide (MgO), it has been discovered that increasing the amount, e.g., weight percent, of CaO and decreasing the MgO by substantially the same amount (weight percent) results in glass having lowered melting and forming temperatures without substantially changing the bending and annealing temperatures of the glass. Also it has been discovered that decreasing the amount of MgO in the glass and increasing the amount of at least two or more of CaO, R2O (Na2O and K2O), Al2O3 and/or SiO2 reduces the liquidus temperature. The reduction and concomitant increase in amounts of these materials is performed without adversely impacting the corrosiveness of the glass melt. Additionally depending on which temperature of the melting, softening, and/or liquidus temperature is to be impacted the concomitant increase in the amounts of which of the two or more of the aforementioned materials can be effected. For instance the alteration of the softening point of the glass can be effected to match glass compositions from different melters to achieve a common softening point for any subsequent bending and annealing operations. In this instance the amounts of CaO and R2O and/or Al2O3 and/or SiO2 can be increased so that the total of the increase across two or more of these materials equals the decrease in the amount of MgO.
In another aspect of the invention, a method of lowering the melting and forming temperatures of a glass composition includes replacing at least some of the CaO and/or MgO of the glass composition with a metal oxide whose metal ion has a lower field strength than Ca++ and/or Mg++, e.g., Ba++ or Sr++.
A glass composition having advantageous properties for flat glass manufacture is also provided. In one embodiment, the glass composition has a melting temperature in the range of about 2570xc2x0 F. to about 2590xc2x0 F. (1410xc2x0 C. to about 1421xc2x0 C.) and a forming temperature in the range of about 1850xc2x0 F. to about 1894xc2x0 F. (1010xc2x0 C. to about 1034xc2x0 C.). The glass composition has a bending temperature in the range of about 1300xc2x0 F. to about 1350xc2x0 F. (704xc2x0 C. to about 732xc2x0 C.) and an annealing temperature in the range of about 1016xc2x0 F. to 1020xc2x0 F. (547xc2x0 C. to 549xc2x0 C.).
In another aspect of the invention the glass composition has a reduced liquidus temperature without increasing the alkali components to make the composition too corrosive. In such an aspect the glass composition has a melting temperature in the range of about 2510xc2x0 F. to about 2650xc2x0 F. (1376xc2x0 C. to about 1454xc2x0 C.) and a forming temperature in the range of about 1800xc2x0 F. to about 1894xc2x0 F. (982xc2x0 C. to about 1034xc2x0 C.) and a liquidus temperature in the range of about 1780xc2x0 F. to about 1850xc2x0 F. (971xc2x0 C. to 1010xc2x0 C.). The glass composition can have a bending temperature in the range of about 1300xc2x0 F. to about 1350xc2x0 F. (704xc2x0 C. to about 732xc2x0 C.) and an annealing temperature in the range of about 1016xc2x0 F. to 1020xc2x0 F. (547xc2x0 C. to 549xc2x0 C.). In such a glass composition the MgO amount is in the range of about 1 to about 3 weight percent for higher iron containing glasses and of about 0.01 to 0.15 weight percent for lower iron containing glasses. The higher iron containing glasses have an iron content of at least 0.1 and the lower iron containing glasses has an iron content of less than 0.1 weight percent. The CaO+R2O+Al2O3 combined amount can make up for the reduction in MgO when the combined amount is in the range of about 23 to about 29 weight percent.